Which mattress is best for me

I am 5.4 and 11.6st I currently have a king size divan medium/firm pocket spring bed, after around 4 hours of being asleep I wake up with really bad back pain between the bottom of my shoulder blades and my lower back the only way to get rid of the pain is to get up and walk around.
I would like to know which mattress I should buy to improve this?